For gears used in machine tools, construction machines, automobiles, machines for ships, etc. as well as industrial machines such as rolling mills have heretofore been used lubricants having superior scoring properties so that the gears can withstand high pressure exerted on the tooth faces thereof. Further development of lubricants having more improved scoring properties have been continuously made.
Examination of failures of gears has revealed that scoring occurs only limitedly; rather the fatigue phenomenon such as pitting, is mainly responsible for various troubles. This fatigue phenomenon is also observed for bearings. In connection with the fatigue phenomenon, it is generally known that the fatigue life is prolonged when the viscosity of the lubricant used is increased, whereas the life is reversely shortened when those lubricants containing polar compounds such as extreme pressure additives are used.
Accordingly, as to a procedure for improving the fatigue life of gears, bearings, etc., there have been known no other effective procedures than that in which the amounts of extreme pressure additives, etc. being added are controlled as low as possible, and lubricants having a high viscosity are employed.